The long-term goal of these investigations is to determine the genetic events that underlie commitment of lymphocytes to one or the other of the several functional lineages. The negative control of immunoglobulin and other B cell-specific genes by a T lymphoma suggests a mechanism whereby the genetic programs of these two types of lymphocyte are kept distinct. The experiments described in this proposal seek to further define the Ig- derived DNA sequences that mark a gene for silencing by the T lymphoma and to identify the mechanisms involved. This involves mutational analyses of DNA fragments that promote gene expression in Ig-producing myelomas but silence genes in myeloma x T lymphoma cell hybrids. Previous experiments by this laboratory have implicated the octamer motif (ATTTGCAT) in gene silencing. The hypothesis that octamer-binding factors are differently modified yielding opposite activities in the myeloma and T lymphoma, respectively, will be approached biochemically. Efforts to clone the factor(s) involved will continue: both a gene transfer and a subtraction library approach will be taken. Somatic cell hybrids across species will be used to map T cell-mediated gene-extinction activity to a chromosome. Preliminary experiments suggest that the transcriptional silencing activity of T cells affects gene expression within the T cell itself, suggesting a role for this activity in the normal development of T lymphocytes. These experiments will be extended, using a number of different reporter gene constructs that will help map the target of this activity. A collaboration with Dr. Dimitris Kioussis will be initiated in which the repressive effects of the IgH enhancer in developing thymocytes will be studied using transgenic mice. Finally, completed experiments show coordinate control of several B cell- specific genes in myeloma x T lymphoma hybrids; analyses of T cell- specific genes in these hybrid lines will be undertaken to determine whether the "on" and "off" signals for the B cell-specific genes have reciprocal meaning for T cell-specific genes.